The present invention relates to a method for operating an injection valve of an internal combustion engine, whereby the metering of fuel is adjustable in particular by varying the stroke of the nozzle needle of the injection valve. The present invention also relates to a computer program and a control device for carrying out the method, and an internal combustion engine with a control device of this type.
High-pressure fuel injection valves for direct fuel injection in the context of internal combustion engines are generally known. Specific types of high-pressure fuel injection valves 300, as shown in FIG. 3, are also known, in particular, with which it is possible to meter the fuel to be injected into the combustion chambers of the internal combustion engine via the opening time of the valve, and, in particular, by varying the stroke of valve needle 330. With these valves, nozzle needle 330 is actuated, e.g., directly with the aid of a piezoelectric actuator 320. Valves of this type are suited in particular for transmitting very short injection pulses and a plurality of injection pulses during a single working cycle, i.e., “multiple injection”, within a very short time window. An example of a nozzle used with a valve of this type is the outwardly opening nozzle, i.e., the “A” nozzle.
In the related art, the needle stroke with the injection valves described—which allow the nozzle needle stroke to be varied—is limited in terms of high values and low values for the valve stroke using diverse restrictions:
The needle stroke is limited in terms of high values, for example, by                the possibilities for realizing a required small amount, because the nozzle surface and the fuel system pressure are specified by the combustion method used and are variable, although in a limited manner;        the size of piezoelectric actuators 320 that can be inserted, and by their physical properties, such as their stroke capacity, the amount of force they can apply, their ability to accelerate, etc.; and by        the capacity of the end stage of the control device (power dissipation, installation space).        
In contrast, the needle stroke under rated operating conditions of the internal combustion engine, i.e., the nominal needle stroke in terms of small stroke values, i.e., in the form of a minimum needle stroke, is limited by                the need to ensure an adequate scavening effect.        
The purpose of this minimum needle stroke is to ensure that, under rated operating conditions of the internal combustion engine, when the injection valve is operated primarily with the nominal stroke, a valve is prevented from closing incompletely or jamming open due to the presence of particles in the cross section of the injection nozzle.
The assurance of an adequate scavenging effect is particularly important because it prevents the risk of damage caused by a gasoline-filled cylinder. Damage caused by a gasoline-filled cylinder occurs when the cross section of the injection valve is blocked by dirt particles, thereby preventing the injection valve from closing completely. In this case, fuel is delivered continuously into the cylinder whose injection valve is jammed. Since fuel can be considered to be an incompressible medium, the piston motion is hindered or blocked when the amount of fuel injected exceeds the compression volume of the cylinder. If other cylinders in the internal combustion engine are functioning properly at this time, they exert—via the crankshaft—very strong forces on the connecting rod and the piston of the blocked cylinder, which usually results in irreparable engine damage, particularly in the form of the connecting rod breaking and a damaged cylinder.
As described above, soiling of the nozzle with dirt particles is prevented via specification of the minimum needle stroke under rated operating conditions.
This specification of the minimum needle stroke applies only for the rated operating conditions of the internal combustion engine, however. It does not rule out the fact that the injection valve must also be closed completely at times between individual injection impulses and/or strokes of the nozzle needle. This applies in particular for operating states with a low fuel requirement, e.g., during idle, when the nozzle needle is actuated for a prolonged period of time not with complete strokes, as is the case under rated operating conditions, but only with partial strokes. In these operating states of the internal combustion engine, an adequate scavenging effect is no longer ensured, due to the partial-stroke actuation. This results in a greatly increased risk that the injection nozzle will become soiled and make it more difficult to detect such soiling.
During idle operation in particular, two injection cycles of a cylinder can be separated by a long period of time. As a result, only relatively few injection events are available for diagnosing the undesired jammed-open operating state of the nozzle needle; this makes diagnosis of this operating state less reliable overall.